While computing window functions, we can take advantage of the general use case where they specify the same PARTITION and ORDER BY criteria.
We already take advantage of this fact by only sorting the internal tmp table as few times as possible. Filling the window function values within the internal tmp table can be performed during one pass / sort as well. Currently we do a pass for each window function.
In order to do this, we must refactor the computation code to be aware of multiple functions during a scan.